The present invention relates to a variable emission angle strobe light in which a light emission angle is varied by varying the position of a light source relative to a reflector and a method for controlling a movement of the light source in the strobe light corresponding to the focal length set in a camera lens. More particularly, the invention relates to a variable emission angle strobe light which highly accurately and inexpensively realizes the setting of the light emission angle corresponding to the focal length set in the camera lens and a method for controlling the movement of the light source in the strobe light.
As a conventional variable emission angle strobe light in which the light emission angle is varied by varying the position of the light source relative to the reflector, a light distribution angle varying device of an electronic flash for cameras (hereinafter, referred to as first prior art) is known, which is disclosed in the Japanese Published Unexamined Patent Application Sho 62-51453.
The light distribution angle varying device according to the first prior art comprises, as shown in FIG. 5A and FIG. 5B, a front reflector 1 fixed to a fixing member (not shown), a rear reflector 2 situated at the bottom of the front reflector 1 and having a light source 3 such as a flash discharge tube, and a driver (not shown) for causing the rear reflector 2 and the light source 3 to reciprocate independently of each other in the direction of arrow A.
When the light source 3 emits light, a subject (not shown) is irradiated with distributed light, i.e. with a luminous flux 3a directly emitted from the light source 3, a luminous flux 3b first reflected by the rear reflector 2 and then traveling toward the center, and a luminous flux 3c reflected by the front reflector 1.
As shown in FIG. 5A, when the rear reflector 2 and the light source 3 move rightward and become away from the front reflector 1, the luminous fluxes 3a, 3b and 3c are condensed as the light source 3 moves toward the bottom of the front reflector 1 (in the direction of arrow B), so that the light distribution condition becomes the one suitable for a narrow angle of view of a telephoto lens (not shown).
As shown in FIG. 5B, the luminous fluxes 3a, 3b and 3c are diffused as the rear reflector 2 having the light source 3 moves from the bottom side toward the open side (in the direction of arrow C), so that the light distribution condition becomes the one suitable for a wide angle of view of a wide-angle lens (not shown). With this arrangement, only by slightly moving the light source 3, the emission angle of the light emitted by the light source 3 is varied in correspondence with the angle of view of the lens being used.
For example, in a camera for 35 mm standard film, in order to obtain the light emission angle corresponding to the angle of view of a camera lens with a focal length range of 24 mm to 85 mm, according to the device of the above-described structure of the first prior art, the light source 3 is moved approximately 4 mm relative to the reflector 1. On the contrary, in another very typical conventional arrangement (hereinafter, referred to as second prior art), a Fresnel lens is disposed in front of the reflector, and the light source and the reflector are integrally moved relative to the Fresnel lens. In this arrangement, in order to obtain the light emission angle corresponding to a photographic image plane of the camera lens having a focal length range of 24 mm to 85 mm, the light source is necessarily moved approximately 20 mm. Compared therewith, the device of the first prior art is capable of controlling the light emission angle with an extremely small movement amount of the light source. As a result, it is expected that the first prior art produces an advantage of largely reducing the size of the strobe light.
In the light distribution angle varying device of the arrangement according to the first prior art, however, since the light source 3 is moved relative to the reflector 1, a slight deviation of the movement amount of the light source 3 leads to a great variation in light emission angle. For this reason, the device intrinsically has a problem that the operation to move and stop the light source 3 is necessarily controlled with extremely high accuracy.
Specifically, as to the amount of change in corresponding focal length when an element such as the reflector to be moved is moved 1 mm, in the case of the arrangement according to the second prior art, the amount of change is (85-24)/20=3.05 mm. On the other hand, in the case of the arrangement according to the first prior art, the amount of change is (85-24)/4=15.25 mm. Thus, in the first prior art, the focal length range corresponding to a 1-mm movement of the element to be moved is extremely wide. In other words, a slight movement of the light source 3 greatly changes the corresponding focal length, so that the positioning is necessarily controlled with extremely high accuracy.
The variation in guide number caused when the element to be moved is moved 1 mm is approximately 0.1 EV in the case of the second prior art which integrally moves the light source and the reflector. On the contrary, in the case of the first prior art which moves the light source 3, the variation is approximately 0.5 EV. Thus, in view of the guide number variation characteristic, the positioning of the light source is necessarily controlled with high accuracy.
As a result of an experiment carried out by the inventors of the present invention, it was confirmed that, for example, when a flash discharge tube with a total length of approximately 50 mm was used as the light source, the positioning of the light source relative to the reflector is necessarily made with an accuracy of .+-.0.1 mm. Therefore, in the case where the movement of the light source is automatically controlled, it is considered to use a stepping motor as the driving unit of the driver for the automatic control. The stepping motor is capable of extremely accurately and instantly moving the light source by a predetermined distance and stopping it.
However, even if the light source is accurately moved and stopped by use of the stepping motor, in view of mass production, it is very difficult to assemble the reflector and the light source so that the accuracy of the positional relationship between the reflector and the light source is .+-.0.1 mm. That is, not only the emission angle at a reference position of the light source in the light distribution angle varying device but also the emission angles at moved positions of the light source deviate from the design values. Therefore, the above-described arrangement intrinsically has a defect that it is impossible to realize an accurate light distribution condition corresponding to the angle of view of the lens being used.
In order to cope with the defect, for example, a position detection unit is provided which is capable of detecting a predetermined position of the light source within a movement range of the light source. It is considered to arrange the light distribution angle varying device so that, by using the position detected by the position detection unit as the reference position, the movement position of the light source is controlled based on the number of steps of the stepping motor from the reference position.
In this arrangement, the light source is returned to the reference position before being moved. By assembling the device so that the light source is situated within the movement range thereof, the movement of the light source is desirably controlled regardless of the assembly accuracy. For example, the reference position relative to the reflector is adjusted so as to be correctly recognized as the absolute position corresponding to a predetermined light distribution condition and the adjusted reference position is set, so that the operation to move the light source can be carried out in correspondence with the control varying operation of the light distribution angle.
The adjustment and setting method to accurately recognize the reference position will be described hereafter. For example, the position detection unit is arranged so that the position detectable as the position of the light source is variable within the movement range of the light source. The light source is also arranged so that the light distribution characteristic of the light source is measurable. A method is considered to adjust the position of the light source detected by the position detection unit so that the light distribution characteristic obtained when the light source emits light after being moved to a currently detectable position is a predetermined reference light distribution characteristic (light distribution characteristic at the reference position).
However, according to this method, a light distribution measuring device which is large-sized and expensive is necessarily provided in the manufacturing process and the structure of the position detection unit is complicated. In addition, some time is necessary for the adjustment. Thus, the method has defects which renders it disadvantageous in mass productivity and manufacturing cost.